Cytochrome P450 enzymes catalyze a variety of reactions, such as detoxication of drugs and carcinogens, biosynthesis of steroid hormones, vitamin activation, etc. Electrons, required for oxidation of substrates, are being delivered to P450s by either iron-sulfur proteins or FAD/FMN-containing NADPH-cytochrome P450 reductase. How P450s interact with their redox partners and what is the mechanism of electron transfer are the key questions in mechanism and function of P450 enzymes. Cytochrome P450BM-3, a soluble fusion protein consisting of both the heme- and reductase domains on a single polypeptide chain, is the best model for studying P450-P450 reductase interactions. We crystallized a 75 kDa fragment of this enzyme, consisting of both the heme- and FMN-binding domains. Large unit cell and poor diffraction pattern, obtained using in-house X-ray source, do not allow us to refine this structure.